MEASUREMENT AND COMPENSATION OF MICROPHONICS IN CW-OPERATED TESLA-TYPE CAVITIES∗

When operating TESLA type cavities in cw-mode, mi-crophonic detuning becomes the most prominent source for fluctuations in their resonant frequency. Such fluctua-tions increase the required RF power-reserve and the ampli-tude and phase-jitter of the accelerated electron beam. It is therefore highly desirable to be able to compensate micro-phonics. For the purpose of frequency adjustment various tuning systems have been developed (DESY, Saclay, KEK, INFN) which all change the cavity dimensions by applying an external force on the cavity-liquid He tank system, do-ing so either rapidly with a piezo-actuator or slowly but at a larger overall range with a stepper motor. Two tuners were tested: the Saclay I tuner and the Saclay II tuner. They were compared with respect to their usability for microphonics compensation. Microphonics were characterized and fur-thermore compensated using a feedback algorithm for slow frequency drifts caused by the cryogenics system up to 1Hz and a feed-forward algorithm for the de-excitation of me-chanical resonances of the cavity-tank-tuner system above 1Hz